METHOD AND APPARATUS FOR REDUCING AN AUDIO HOLE DURING HANDOFF IN A COMMUNICATION SYSTEM

Abstract: A communication system reduces audio holes due timing related to messaging and switching of ports during handoff. Additional resources are added between a base station controller (BSC) and a mobile switching center (MSC, 123) to aid in information transfer between a target BSC (120) and a source BSC (121) during handoff. By separating the timing problems related to each of the messaging and the switching of ports, the two are not able to compound one another, and the audio hole is subsequently reduced.

Description: METHOD AND APPARATUS FOR REDUCING AN AUDIO HOLE DURING HANDOFF IN A COMMUNICATION SYSTEM Field of the Invention The field of the invention relates generally to communication systems and more particularly to communication handoff within communication systems. Background of the Invention Communication systems, and typically cellular radiotelephone systems, employ a handoff of communication of a mobile unit from one communication channel to another based on certain criterion. When handoff is implemented, a number of events must occur. First, the mobile unit is informed, via a BTS of the cellular radiotelephone system, that its communication with a source base transceiver station (BTS) is undergoing handoff to a target BTS. Second, switching of ports, via an interface coupling the source and target BTS, must be performed to appropriately route the external party to the mobile unit on the target channel. As these two events occur at different locations of the cellular radiotelephone system, there may be a indeterminate time for messages to be sent reliably to the mobile unit and a different indeterminate time for the messages to cause the switching of ports to occur. Although it is possible to bound these times, the potential for these events to not happen simultaneously is great. If the time difference between events is great, a long break in the air-interface occurs. This break is commonly called an audio hole. One solution to overcome the audio hole problem is to utilize a three-party circuit (TPC) to add the audio from the potential sources together during broadcast on source and target channels. This solution has drawbacks, however, since the need for TPC hardware can become quite costly and the use of the TPC places a larger burden (in terms of loading) on the infrastructure equipment responsible for switching. Other means exist, but they typically require the cell-sites to be in communication with one another or for a transcoder (if present) to receive information from both cell-sites. When handoff depends on higher level infrastructure equipment (for example, a mobile switching center or MSC), these means are often not available. Thus a need exists for a communication system which mitigates an audio hole with consideration to both system cost and loading considerations. Brief Description of the Drawings FIG. 1 generally depicts BTSs coupled to an MSC via BSCs in a communication system that may beneficially employ the present invention. FIG. 2 generally depicts in block form a BTS controller (BSC) and transceivers located within the BTSs of FIG. 1 and used to implement the present invention. FIG. 3 illustrates a voice channel controller (VCC) located within the BSC of FIG. 2 and used to control the scanning and voice transceivers found in FIG. 2. FIG. 4 generally depicts a schematic diagram of a receiver which may be employed to demodulate a radio frequency (RF) signal and determine a received signal strength indication (RSSI) of the signal in accordance with the invention. FIG. 5 generally depicts in block form audio hole mitigation in accordance with the invention. Detailed Description of a Preferred Embodiment FIG. 1 generally depicts a communication network which may beneficially employ the present invention. In the preferred embodiment, the communication system is an analog cellular radiotelephone network (AMPS), but any cellular radiotelephone system such as narrowband AMPS (NAMPS), time-division multiple access (TDMA) systems or code-division multiple access (CDMA) systems (or any combination thereof) may beneficially employ the present invention. Cellular radiotelephone networks may incorporate as many different cells as is required to accommodate the coverage area. For example, a cell 100 typically covers only a small portion of the total coverage area in large cellular systems. As the cellular network grows in a particular coverage area, frequency reuse becomes an important parameter that is used to accommodate all the mobiles necessary. Returning to FIG. 1, each cell 100-101, 110-112 has its own base transceiver station (BTS), which is required to communicate to a mobile unit 125. In source cell 100, a source BTS 130 communicates to mobile unit 125 on a radio frequency (RF) signal. Other BTSs 131, 140-142 are target BTSs since they are "targets" for future transfer of communication (handoff). Each of the BTSs 130-131, 140-142 includes a plurality of transmitters and receivers for operating on at least one duplex signaling channel and a plurality of duplex voice channels. One conventional system employs transmitters and receivers of the type described in Motorola Instruction Manual No. 68P81058E05-A published by Motorola Service Publications, Schaumburg, IL., in 1989. FIG. 2 generally depicts transceivers and associated controlling hardware within one BTS coupled to a BSC. BSC 121 typically provides the interface between BTSs 130-131 within cells 100-101 and the MSC 120. Elements of BSC 121 generally include a cell-site controller (CSC) 203, a voice channel controller (VCC) 212, a signaling channel controller (SCC) 206, and an extend multiple port interface (EMPI) (not shown). An identical secondary BSC can be employed to provide BSC redundancy. Together, these elements provide voice channel assignment, signaling channel control, performance monitoring, and other control functions that are used by the BSC 121 to initiate calls, determine channel allocation, and make handoff decisions. BSC 121 also includes a enabler 201 and a switch matrix 200 appropriately coupled to MSC 123, CSC 203 and voice audio circuits of BTS 130 for reduction of an audio hole in accordance with the invention. Operation of these elements are subsequently explained below. VCC 212 controls the primary voice transceivers 222-225 and the scanning receiver 221. FIG. 3 generally depicts a block diagram of a VCC 212 which may be employed to implement the present invention. The central processing unit (CPU 306) may be a microprocessor such as MC6809 available from Motorola, Inc. This CPU 306 is used to control the primary voice channel transceivers 222-225 and scanning receiver(s) 221 in accordance with programmed steps stored in RAM and EPROM 312. In a nonredundant configuration of the present invention, VCC 212 may control up to 30 channels of radio equipment and one scanning receiver employing SSDA 310, voice transceiver interface 320, and scanning receiver interface 322. Signaling data encoding and decoding is controlled via SSDA 308 and signaling encoding and recovery interface 318. Transceivers are selected by enable drivers 324 which is controlled by CPU 306 via peripheral interface adaptors (PIA) 314. In the redundant configuration requiring two VCCs, each transceiver (being dual ported) can communicate with both VCCs. Under normal operating conditions, each VCC actively controls half of the channels while exchanging control and status messages with the other half. Interface to a redundant CSC is accomplished via ADLC 304. A transmitted signal from a mobile unit 125 enters one of voice transceivers 222-225 via a matrix RF board (not shown). FIG. 4 generally depicts a schematic diagram of receiver hardware employed to demodulate the transmitted signal from the mobile unit 125 and determine a signal quality value or signal strength indication (SSI) of the signal. Referring to FIG. 4, the receiver has as input an even and odd signal for diversity reception purposes. An even mixer /IF board 405 accepts the even signal while an odd mixer/IF board 410 accepts the odd signal. Both mixer/IF boards 405,410 have an input from a side-step synthesizer 400 which acts as a first local oscillator (LO) for mixing purposes. An injection /amplifier board 415 provides another input to each mixer/IF board 405,410 and acts as a second LO for mixing purposes. The IF ICs 445, 450, which in the preferred embodiment are part number TDA 1576 available from Signetics, provide limiting, quadrature detection, and amplification to yield an audio output signal which is further amplified and fed to a audio control board 420 for processing. SSI detection is performed by circuitry internal to the IF ICs 445, 450 and by discrete SSI detection range extension circuitry. The outputs of IF ICs 445, 450 individually provide linear SSI signals. These SSI signals are fed to the audio control board 420 where they are used to drive a diversity switch 425, which in turn is used to enable/ disable an audio switch 430. Output from the audio switch 430 is the audio which represents the communication from the mobile unit 125. This audio is output to the BSC 121 via conventional phone lines. Each BTS within a particular cell in a cellular network is coupled to a base-station controller (BSC) 120, 121. BSCs 120, 121 are coupled via a mobile switching center (MSC) 123 which provides switching capability at a higher level in the overall architecture of the cellular radiotelephone system. In the preferred embodiment, MSC 123 is a switching element while BSCs 120, 121 are sub-switching elements. BSCs 120, 121 serve as a junction point between BTSs in a cellular network. For example, as mobile unit 125 moves throughout source cell 100, BTS 130 continuously monitors the received signal strength indication (RSSI) of the mobile unit 125. As mobile unit 125 moves away from BTS 130, the RSSI of the mobile unit 125 eventually falls below a transfer threshold. Once it is known that a handoff is desired, BSC 121 will either immediately initiate the handoff procedure or it will solicit signal strength measurements for adjacent cells. Signal strength measurements are made if it is unclear whether there is an available target BTS that can serve mobile unit 125 with adequate signal quality (such as in an analog signaling type). For analog signaling types, BSC 121 determines which neighbor cells a -- appropriate to request strength measurements from. To start the handoff procedure, source BTS 130, via BSC 121, sends a Signal Strength Measurement Request message to MSC 123 (or an adjacent BSC) which allows BSC 121 to interrogate adjacent cell sites to determine if mobile unit 125 can be better served by an adjacent cell. When MSC 123 receives this message, it examines the list of cells being requested to perform a strength measurement, and then forwards the request over the A interface (the link coupling a MSC to a BSC) with the appropriate subset of the candidate cell list to each appropriate BSC that has domain over those cells. For example, the. appropriate subset of the candidate list for BSC 120 would be cells 110-112. BTSs 131, 140-142 receive the Strength Measurement Request message, receive the transmission of the mobile unit 125 and, if the handoff criterion is met, target BTSs 131, 140-142 will send a response to the BSC 121 noting such. The handoff criterion is based on a signal strength measured at target BTSs 131, 140-142, plus some hysteresis value, when compared to a signal strength measured by source BTS 130. The hysteresis value is used by neighboring BTSs 131, 140-142 to take into account the physically different locations of the target BTSs 131, 140-142 with respect to source BTS 130. At this point, the BSC 121 determines which of the target cells 101, 110-112 is a better candidate for handoff. Important to note is that MSC 123 can also make the handoff decision. Continuing, if for example, target BTS 131 in cell 101 measures a RSSI plus hysteresis higher than that of target BTS 140 in cell 110, target BTS 131 in cell 101 would be a better candidate for handoff. In this scenario, the BSC 121 sends a response to target BTS 131 in cell 101 to determine if there is an idle channel available for communication. If there is, target BTS 131 sets-up the channel, begins transmission and notifies the BSC 121 of the channel and supervisory audio tone (SAT) information. BSC 121 relays the channel and SAT information to source BTS 130, source BTS 130 instructs mobile unit 125 to tune to the new channel and to chang SAT, and mobile unit 125 begins to communicate to the target BTS 131 in cell 101. At this point, mobile unit 125 is determined to have arrived in cell 101 by the use of its new SAT. In digital cellular radiotelephone systems such as TDMA and CDMA, mobile unit 125 is capable of assisting in the handoff procedure by a process called mobile assisted handoff (MAHO). During MAHO, mobile unit 125 makes measurements of surrounding neighbor cells and reports the results of the measurements to source BSC 121 (via source BTS 130). In these new digital cellular radiotelephone systems, the handoff criterion is not signal strength but is typically a measure of the bit error rate (BER) of the signal, but may be any qualitative measure of the signal. At this point, source BSC 121 (or MSC 123) determines the best target for handoff utilizing the particular qualitative measure employed, and initiates the handoff procedure described above. Handoff across MSC 123 is the most likely scenario to produce an audio hole due to the timing of handoff events such as messaging and switching. FIG. 5 generally depicts in block form audio hole mitigation in accordance with the invention. As depicted in FIG. 5, resources at a BSC, for example BSC 121, are utilized to reduce handoffs by adding additional resources between BSC 121 and MSC 123 to carry handoff information from a target BTS, for example BTS 140, to source BTS 130. As an example, BSC 121 (the source BSC) would have means in block 500 of BSC 121 to control the coupling to MSC 123 based on predetermined criteria. In the preferred embodiment, block 500 generally comprises switch matrix 200 and enabler 201 of FIG. 2. Enabler 201 controls coupling based on predetermined criteria which include, but are not limited to, the performance of a soft handoff, the connection of three parties (a three party switch), and the detection of whether a mobile unit is on (the target) channel. In the connection of three parties situation, enabler 201 is comprised of a three party switch. In the soft handoff and detection of whether a mobile unit is on channel situation, enabler 201 is comprised of a digital signal processor (DSP), which in the preferred embodiment is a DSP from the 56000 family of DSPs manufactured by Motorola. In a typical handoff across MSC 123, messaging would occur between source BTS 130 and target BTS 140 via MSC 123. At a specified time after messaging has begun, MSC 123 would make a hard switch as shown by the dotted line in FIG. 5, thus connecting the source port (from the PSTN) to mobile unit 125 via target BSC 120 (and target BTS 140). Switching of ports in the MSC during messaging, however, contributes to the generation of the problematic audio hole. In accordance with the invention, rather than a hard switch from source BSC 121 to target BSC 120 when handoff across MSC 123 occurs, MSC 123 is instructed to first couple (by setting up a trunk) source BSC 121 and target BSC 120). Source BSC, by implementing one of the means mentioned above, provides audio hole reduction by determining when handoff has occurred (i.e., when mobile unit 125 is off the source channel) and informing MSC 123 to make the dotted line connection in FIG. 5 to complete the handoff. In this manner, the timing problems related to messaging and port switching during handoff across MSC 123 which greatly contribute to the audio hole are separated. Consequently, the effects to a user of the audio hole is mitigated since the timing related to messaging and switching do not compound upon one another. As stated above, three criteria to control coupling to MSC 123 are provided. Enabler 201 of block 500 of FIG. 5 would comprise a three party switch. In this scenario, a three party switch would be used to add the audio paths of BTS 130 and BTS 140 together. This would cause voice audio, in the form of pulse code modulated (PCM) data, to be sent across the three party connection and to target BTS 140 via the coupled path in MSC 123 in accordance with the invention. If the means to reduce the audio hole is a detection of information from the target BSC 120 rather than the source BSC 121, the same connection is made in MSC 123, but instead of voice information being passed, target BSC 120 sends information received to source BSC 121. In this situation, enabler 201 of block 500 comprises a DSP. Based on a response from CSC 203, enabler 201 determines handoff is complete. At this time, BSC 121 commands MSC 123 to switch to the dotted line connection as shown in FIG. 5 to connect the source port to mobile unit 125 on the target BTS 140. If the system were a CDMA communication system, soft handoff would be employed. During soft handoff, mobile units support physical RF connections with multiple cells. This scheme allows the mobile unit to receive simultaneous transmissions from multiple cells. On the uplink (transmission from mobile unit to BTS), multiple cells may receive the transmission from a mobile unit and use the results of the reception at multiple sites if a site selection diversity reception scheme were employed. During soft handoff, transmissions from all cells in a CDMA communication system are synchronized in time. In a soft handoff scenario, the enabler 201 of block 500 would comprise a DSP. When enabler 201 determines that the original source channel is no longer contributing valuable information to the handoff process, the soft handoff has been completed, BSC 121 commands MSC 123 to switch to the dotted line connection as shown in FIG. 5 to connect the source port to mobile unit 125 on the target BTS 140. While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. • For example, in alternate embodiments, target BSC 120 could provide an equivalent to block 500 as well. In this case, MSC 123 would first couple source BSC 121 to target BSC 120, then once the means of block 500 i configured, MSC 123 would change the connections from the source port to the target BSC 120. When the handoff is complete, MSC 123 would be instructed to tear down the link within MSC 123 which couple the two BSCs. In either case (source or target BSC implementation) the links between the BSCs could be fixed so as to off-load the task from MSC 123. In this case, MSC 123 would be informed only when the handoff has been completed in the source BSC implementation, or when the target BSC 120 is ready to be connected to the source port for the target BSC implementation. In alternate embodiments, the scheme could be utilized to reduce audio holes when handoff occurs across multiple MSCs coupled to one another. In this scenario, each MSC would be required to have the capability of switching to the path to reduce the audio hole as shown in FIG. 5. After handoff is complete, the appropriate MSC would connect a mobile unit to the source port while the other MSC maintains the link (or switching between the two MSCs can be done at the PSTN). In still another embodiment, the switching element (the MSC 123 equivalent) may be the PSTN and the sub-switching elements (the BSCs 120, 121 equivalents) could be MSCs in systems which do not require BSCs. What I claim is:

Claims: Claims 1. A method of reducing an audio hole during handoff in a communication system, the method comprising the steps of: determining when a handoff across a switching element is to occur; coupling, within the switching element, a first sub- switching element to a second sub-switching element; determining, within one of the first and second sub- switching element, when messaging for handoff has been substantially completed; and coupling, within the switching element, a source port to the second sub-switching element based on the substantial completion of handoff messaging. 2. The method of claim 1 wherein the switching element further comprises one of either a mobile switching center (MSC) or a public switched telephone network (PSTN). 3. The method of claim 1 wherein the sub-switching element further comprises one of either a mobile switching center (MSC) or a base station controller (BSC). 4. A method of reducing an audio hole during handoff in a communication system, the method comprising the steps of: determining when a handoff across multiple switching elements is to occur; coupling, between each switching element, a first sub- switching element to a second sub-switching element; determining, within one of the first and second sub- switching elements, when messaging for handoff has been substantially completed; and coupling, within one of the switching elements, a source port to the second sub-switching element based on the substantial completion of handoff messaging. 5. A communication system for reducing an audio hole during handoff, the communication system comprising: means for determining when a handoff across a switching element is to occur; means, within the switching element, for coupling a first sub-switching element to a second sub-switching element; means, within one of the first and second sub-switching element, for enabling the coupling to the switching element based on predetermined criteria; and means, within the switching element, for coupling a source port to the second sub-switching element based on a substantial completion of messaging related to handoff. 6. The communication system of claim 5 wherein said means for enabling further comprises one of either performance of a soft handoff, connection of three parties via a three party switch, and the detection of whether a mobile unit is on channel. 7. A communication system for reducing an audio hole during handoff, the communication system comprising: means for determining when a handoff across multiple switching elements is to occur; means, between each switching elemt t, for coupling a first sub-switching element to a second sub-switching element; means, within one of the first and second sub-switching element, for enabling the coupling to one of the switching elements based on predetermined criteria; and means, within one switching element, for coupling a source port to the second sub-switching element based on a substantial completion of messaging related to handoff. 8. The communication system of claim 7 wherein said means for enabling further comprises one of either performance of a soft handoff, connection of three parties via a three party switch, and the detection of whether a mobile unit is on channel.

Inventors: SPEAR STEPHEN LEE, SPEAR, STEPHEN, LEE
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1994
Main classification: H04Q7
IPC codes: H04B7, H04W36, H04W56